1. Field of the Invention
The present invention relates to a differential amplifier circuit, and more specifically to a differential amplifier circuit configured to be selectively brought either into an active condition or in an inactive condition by a control signal.
2. Description of Related Art
One typical conventional differential amplifier circuit configured to be selectively brought either into an active condition or in an inactive condition by a control signal, comprises an input stage composed of a differential amplifier and an output stage composed of an emitter follower. When the differential amplifier circuit should be maintained in the active condition, the input and output stages are maintained in an active condition by the control signal, so that an input signal is amplified by the input stage and outputted through the output stage. On the other hand, if the differential amplifier circuit should be maintained in the inactive condition, the input and output stages are brought into an inactive condition by the control signal, so that the output stage supplies no output current regardless of a level of the input signal supplied to the input stage.
In the above mentioned conventional amplifier circuit, when the input and output stages are brought from the active condition into an inactive condition by the control signal, the output stage is not immediately cut off. Specifically, the output stage continues to flow the output current for a time period of several tens microseconds to several hundreds microseconds after the control signal is brought from a condition indicating that the differential amplifier circuit should be maintained in the active condition to another condition indicating that the differential amplifier circuit should be brought into the inactive condition.
In other words, the conventional amplifier circuit is disadvantageous in that it has a relative long transition time from the active condition to the inactive condition.